My recollection from ye olde ARPANET days about why we capitalized it was the concept that "the" capital-I Internet was to refer to that One Big (TCP/IP) Network of networks, whereas "an" (lower-case-i) internet was any of those random assemblages of just-as-commonly-talked-about networked networks (not themselves connected to THE Internet, and not necessarily using TCP/IP). Of course, now we really only think of one network (and rarely even think of it as a network of networks, per se -- CSNet anyone? Bitnet? Tymenet?), so there isn't really much use for saying "an" internet these days. "The" Internet has taken over the meaning of the word to such an extent that if you did say "an internet", people would probably think you mean, like, another globe-spanning "the Internet" on some other planet or something. :) Or they'd think "an internet" is an adjective, followed by a noun, say, "an internet connection," and if the noun wasn't there, it wouldn't parse right... So at this point it doesn't matter; now "internet" == "Internet."

I just hope people don't start leaving out the "the"!! :) "I'm on internet" still sounds to me like a confused grandma. (As dissonant as "I'm on phone" or "I'm on bed" or "I'm on way.") Aauuuuuugh! :) :)